Design, Setting, and Participants Survey of 4115 adult men and women conducted in 1999 and 2000 as part
of the National Health and Nutrition Examination Survey (NHANES), a nationally
representative sample of the US population.

Results The age-adjusted prevalence of obesity was 30.5% in 1999-2000 compared
with 22.9% in NHANES III (1988-1994; P<.001).
The prevalence of overweight also increased during this period from 55.9%
to 64.5% (P<.001). Extreme obesity (BMI ≥40)
also increased significantly in the population, from 2.9% to 4.7% (P = .002). Although not all changes were statistically significant,
increases occurred for both men and women in all age groups and for non-Hispanic
whites, non-Hispanic blacks, and Mexican Americans. Racial/ethnic groups did
not differ significantly in the prevalence of obesity or overweight for men.
Among women, obesity and overweight prevalences were highest among non-Hispanic
black women. More than half of non-Hispanic black women aged 40 years or older
were obese and more than 80% were overweight.

Conclusions The increases in the prevalences of obesity and overweight previously
observed continued in 1999-2000. The potential health benefits from reduction
in overweight and obesity are of considerable public health importance.

Figures in this Article

Data from the Third National Health and Nutrition Examination Survey
(NHANES III; 1988-1994) showed that the prevalence of obesity, defined as
a body mass index (BMI) of 30 or higher, had increased by approximately 8
percentage points in the United States after being relatively stable from
1960 to 1980.1,2 Since those data
were published, additional reports from other sources have suggested that
these trends are continuing.3- 6 However,
those reports from the Behavioral Risk Factor Surveillance System (BRFSS)
and the Harris Poll have limitations because they are based on self-reported
weight and height. Obesity prevalence estimates based on self-reported data
tend to be lower than those based on measured data.4 For
example, the BRFSS showed a prevalence of obesity of 12% to 14.4% during 1991
to 19944; the corresponding NHANES estimate
of 22.5% for 1988 to 19942 was more than 50%
higher than the BRFSS estimates. National examination survey data based on
measured weight and height data provide the best opportunity to track trends
in weight in the United States. In this article we report the results from
the latest NHANES data from 1999-2000 regarding population trends in obesity
and in the frequency distribution of BMI.

METHODS

Surveys

The NHANES program of the National Center for Health Statistics, Centers
for Disease Control and Prevention, includes a series of cross-sectional nationally
representative health examination surveys beginning in 1960. Each cross-sectional
survey provides a national estimate for the US population at the time of the
survey, enabling examination of trends over time in the US population. In
each survey a nationally representative sample of the US civilian noninstitutionalized
population was selected using a complex, stratified, multistage probability
cluster sampling design. Previous national surveys include the first National
Health Examination Survey (NHES I, 1960-1962) and the first, second, and third
NHANES surveys (NHANES I, 1971-1974; NHANES II, 1976-1980; and NHANES III,
1988-1994).7- 10

Beginning in 1999, NHANES became a continuous survey without a break
between cycles. The procedures followed to select the sample and conduct the
interview and examination were similar to those for previous surveys. This
report is based on data for 4115 adult men and women from the first 2 years
of the continuous NHANES (1999-2000). Two or more years of data are necessary
to have adequate sample sizes for subgroup analyses.

BMI and Obesity

Weight and height were measured in a mobile examination center using
standardized techniques and equipment. Body mass index was calculated as weight
in kilograms divided by the square of height in meters. For adults, overweight
was defined as a BMI of 25.0 or higher, obesity as a BMI of 30.0 or higher,
and extreme obesity as a BMI of 40 or higher.2 These
definitions are consistent with those of the National Heart, Lung, and Blood
Institute and the World Health Organization.11,12

Data Analysis and Statistical Methods

Statistical analyses were carried out using SAS for Windows software
(SAS Institute, Cary, NC) and SUDAAN software (RTI, Research Triangle Park,
NC). For all surveys, sampling weights had been calculated that took into
account unequal probabilities of selection resulting from the sample design,
from nonresponse, and from planned oversampling of certain subgroups. All
analyses took into account differential probabilities of selection and the
complex sample design. Standard errors were calculated with SUDAAN using Taylor
series linearization for NHANES III.13 For
NHANES 1999-2000, SEs were calculated using the delete 1 jackknife method,13 partitioning the sample into 52 sampling units and
forming 52 replicates by deleting one unit at a time. Statistical hypotheses
were tested univariately at the .05 level using a t statistic.
To adjust for multiple comparisons when 3 racial/ethnic groups were compared,
the Bonferroni method was used. For graphical comparison, the frequency distributions
of BMI from both surveys were smoothed using a nonparametric smoothing algorithm,
based on sequential calculations of running medians for groups of adjacent
points.14

RESULTS

The prevalence of obesity (BMI ≥30) during 1960 to 2000 in the United
States by age and sex categories for those aged 20 to 74 years is shown in Table 1. For surveys up through NHANES
II, data were available only for respondents younger than 75 years. The prevalence
of obesity was relatively constant from 1960 to 1980, then increased as reported
by NHANES III in 1988-1994. The most recent data, from NHANES 1999-2000, show
further increases for both men and women and in all age groups. The increases
from NHANES II to NHANES III were statistically significant in all sex-age
groups. Statistically significant increases also occurred from NHANES III
to NHANES 1999-2000, except for the increase for men aged 40 to 59 years,
which was not statistically significant but showed the same trend. The increases
between NHANES III and NHANES 1999-2000 were almost as large as the increases
between NHANES II and NHANES III and were not significantly different.

Table Graphic Jump LocationTable 1. Trends in the Age-Adjusted and Age-Specific
Prevalence of Obesity for Adults Aged 20-74 Years, 1960-2000*

A more detailed examination of trends by age over a broader age range
between NHANES III and NHANES 1999-2000 is possible because both surveys had
no upper age limit. The prevalence of obesity for both surveys for all adults
and by sex and 10-year age groups is shown in Table 2. Increases in the prevalence of obesity occurred for both
men and women and in all age groups. Because the SEs are relatively large,
particularly for NHANES 1999-2000, the differences are not always statistically
significant, but the trends are similar across all subgroups.

The changes in the prevalence of obesity and extreme obesity between
NHANES III and NHANES 1999-2000 by sex and racial/ethnic group for 3 groups—non-Hispanic
whites, non-Hispanic blacks, and Mexican Americans—are shown in Table 3. In each subgroup the prevalence
of both obesity and extreme obesity increased between NHANES III and NHANES
1999-2000. The increases were generally similar across all groups, although
there was a nonsignificant trend for a larger increase in non-Hispanic black
women. For obesity, the increases were not statistically significant for Mexican
Americans, although trends were in the same direction as for the other racial/ethnic
groups. For extreme obesity, the increases were significant for men and women
overall and for non-Hispanic black women. In other racial/ethnic groups, the
increases were not statistically significant, although the trends were in
the same direction.

Table Graphic Jump LocationTable 3. Changes in Age-Adjusted Prevalence
of Obesity and Extreme Obesity by Sex and Racial/Ethnic Group for Adults Aged
20 Years and Older*

More detailed information on the prevalence of overweight and obesity
by age, sex, and racial/ethnic group from NHANES 1999-2000 is shown in Table 4. The prevalence of overweight,
which was 55.9% in NHANES III, increased to 64.5% (P<.001).
The prevalences of overweight and obesity among men varied little by racial/ethnic
group and there were no significant differences. Among women, non-Hispanic
black women had a higher prevalence of both overweight and obesity than did
non-Hispanic white women. For Mexican American women, the prevalence was intermediate
between the other 2 groups and was not significantly different from either
non-Hispanic white women or non-Hispanic black women. Data on extreme obesity
are not shown because the estimates within subgroups were statistically unreliable.

The distribution of BMI in the population was also evaluated. For men
aged 60 to 79 years, the distribution of BMI between NHANES III and NHANES
1999-2000 has shifted to the right (Figure
1), but the shift is greater at the upper percentiles of the distribution,
indicating that the distribution has become more skewed. This pattern was
also seen for men and women aged 20 to 39 years and 40 to 59 years (data not
shown). For women aged 60 to 79 years the shift is more uniform (Figure 1).

COMMENT

These data indicate that the trends in BMI and the prevalence of obesity
previously observed between the 1976-1980 NHANES II survey and the 1988-1994
NHANES III survey appear to be continuing at a similar level in 1999-2000.
Although these increases in obesity observed in NHANES III and NHANES 1999-2000
appear dramatic compared with previous surveys, they may also be viewed as
part of a longer-term trend for increases in body size in affluent and well-nourished
societies. In the United States, mean BMI appears to have been increasing
over a long time, with recent increases perhaps less steep than those earlier.15 Other developed countries are experiencing similar
increases, and less developed countries also show increases in obesity as
they become more affluent.12 As with NHANES
III, the increases seen in NHANES 1999-2000 appear to be occurring in both
men and women, in all age groups, and in all racial/ethnic groups studied.

The findings also reflect the difference in prevalence estimates based
on measured vs self-reported height and weight. The 2000 BRFSS data3 estimate an obesity prevalence of 19.8% among adults
compared with the estimated prevalence of 30.5% in our study. Relatively little
is known about the precise causes of these trends.16- 18 Although
they must reflect energy imbalances in the sense that energy intake must exceed
energy expenditure for weight to increase, the nature of the imbalances is
not clear. Both dietary intake and physical activity are difficult to measure,
and trends in these factors are not easy to evaluate. A more fundamental problem
is to identify the social, economic, and cultural forces leading to energy
imbalance. Advances in technology, changes in work life, the advent of computers,
trends in eating out vs food preparation at home, time pressures, fear of
crime, decreases in tobacco use, and many other factors have been suggested,
but definitive data are lacking that would clearly associate changes in these
factors with the increase in obesity on an individual basis.

The increases in overweight and obesity raise questions about the implications
of these trends for health outcomes. Obesity is a risk factor for many chronic
conditions including diabetes, hypertension, hypercholesterolemia, stroke,
heart disease, certain cancers, and arthritis. Of these conditions, diabetes
may be most closely linked to obesity, and its prevalence appears to have
increased as the prevalence of obesity increased.19 The
increasing incidence of diabetes worldwide is of considerable concern.20 Clinical trials have demonstrated that a structured
lifestyle intervention including dietary change, weight loss, and increased
physical activity can reduce the risk of progressing to diabetes mellitus
from impaired glucose tolerance.21,22

Other conditions, such as hypercholesterolemia and hypertension, declined
between NHANES II and NHANES III at the same time that the prevalence of obesity
was increasing.23,24 Total cardiovascular
mortality and mortality from coronary heart disease and stroke have also declined
over these years.25 Obesity is a risk factor
for these conditions; however, not everyone with these conditions is obese,
and not all obese people have these conditions.26,27 There
are several risk factors other than obesity for most of these health conditions,
and intervening on these other risk factors may be necessary.11,26,27 Changes
in other risk factors might also affect trends in these health conditions.
It is also possible that some of the conditions associated with obesity may
respond to interventions such as change in the fat content of the diet or
increases in physical activity that are not necessarily accompanied by large
changes in body weight.11

Relatively little is known about the prevention and treatment of overweight
and obesity on a population-wide basis.28,29 On
an individual level, structured programs that emphasize lifestyle changes,
including education, reduced fat and energy intake, regular physical activity,
and regular staff contacts with participants, can produce modest long-term
weight loss on the order of 5% to 10% of starting weight.30

It likely will be difficult to reverse the increasing prevalence of
overweight and obesity in the United States. Even as long ago as 1960, almost
50% of men and more than 40% of women were overweight, and 11% of men and
16% of women were obese. As was shown previously for the shifts between NHANES
II and NHANES III,31 the entire distribution
of BMI appears to be affected, with a shift to the right occurring in all
age-sex groups. Thus, these appear to be population-wide changes, not limited
just to the upper portion of the distribution. Although the health implications
of the increases in obesity and the costs and the risks and benefits associated
with treatments and interventions have not been completely elucidated, the
increase in the prevalence of obesity is clear. The potential health benefits
from reduction in overweight and obesity are a matter of considerable public
health importance.

NHLBI Expert Panel on the Identification, Evaluation, and Treatment
of Overweight and Obesity in Adults. Clinical guidelines on the identification, evaluation, and treatment
of overweight and obesity in adults: the evidence report. Obes Res.1998;6(suppl 2):51S-209S.

Costa DL, Steckel RH. Long-term trends in health, welfare, and economic growth in the United
States. In: Steckel RH, Floud R, eds. Health and Welfare
During Industrialization. Chicago, Ill: The University of Chicago Press;
1997:47-89. A National Bureau of Economic Research Project Report.

Burt VL, Cutler JA, Higgins M.
et al. Trends in the prevalence, awareness, treatment and control of hypertension
in the adult US population: data from the Health Examination Surveys, 1960-1991. Hypertension.1995;26:60-69.

National Cholesterol Education Program. Third Report of the Expert Panel on Detection, Evaluation, and Treatment
of High Blood Cholesterol in Adults (Adult Treatment Panel III). Available at: http://www.nhlbi.nih.gov/guidelines/cholesterol/index.htm. Accessibility verified September 10, 2002.

NHLBI Expert Panel on the Identification, Evaluation, and Treatment
of Overweight and Obesity in Adults. Clinical guidelines on the identification, evaluation, and treatment
of overweight and obesity in adults: the evidence report. Obes Res.1998;6(suppl 2):51S-209S.

Costa DL, Steckel RH. Long-term trends in health, welfare, and economic growth in the United
States. In: Steckel RH, Floud R, eds. Health and Welfare
During Industrialization. Chicago, Ill: The University of Chicago Press;
1997:47-89. A National Bureau of Economic Research Project Report.

Burt VL, Cutler JA, Higgins M.
et al. Trends in the prevalence, awareness, treatment and control of hypertension
in the adult US population: data from the Health Examination Surveys, 1960-1991. Hypertension.1995;26:60-69.

National Cholesterol Education Program. Third Report of the Expert Panel on Detection, Evaluation, and Treatment
of High Blood Cholesterol in Adults (Adult Treatment Panel III). Available at: http://www.nhlbi.nih.gov/guidelines/cholesterol/index.htm. Accessibility verified September 10, 2002.

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